System for monitoring cables

ABSTRACT

System for monitoring high-tension cables in air stretches as well as a use of the same. The system comprises at least one optic fiber fastened to the high tension cable, the optic fiber including at least one Bragg grating with known reflection characteristics, a light source for emitting light within a known range of wavelengths into the optic fiber, and measuring devices for detection of light reflected from the Bragg grating in the fiber and for recognizing light reflected from each Bragg grating based on their known reflection characteristics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system for monitoring high voltagecables in air stretches.

2. Description of the Related Art

Air lines for power supply is subject to mechanical strains from theenvironment such as snow and ice load, lightnings, extreme temperaturesand wind given from terrain and geographical and topographicalconditions. Critical wind strains are extreme wind gusts, vibrationswearing on the end mounts and galloping which may raise from acombination of wind and ice loads and may produce short circuits anddamage on the lines. In general extreme climatic strains may result inpower breaks and substantial repair works with large economicalconsequences. A real time monitoring system especially in inaccessibleareas may provide information about lines and/or masts being subject tocritical loads so that corrective procedures may be performed beforelarger damage occurs.

Conventional methods for registering climatic strains have until nowmainly been based on indirect measurements of the search for variables.Simple ice rigs or a couple of stretches of conductors without voltageis combined with strain detectors to provide a value for icing in therelated area. Optical telescopes are used for measuring vibrations overshort time periods. Video cameras are used for visual inspection ofgalloping in chosen point on the line. Common for all these solutionsare that they do not measure the direct strain on the line, butvariables which in a varying degree is directly related to this. One ofthe reasons for these problems is the handling at the high voltages.These methods depends on local power supply. Optical fibers of quartsglass are immune to electromagnetic fields and the signal propagation istherefore not affected by the high voltage environment. Also the signalsmay be transported over long distances so that one does not have todepend on local power supplies. Supply fibers positioned in a suitablefiber optic cable may easily be winded around a power line in the sameway as fibers used for communication, which now is a well known method.Alternatively the fibers are integrated in a line strand. The mostimportant difference between the existing and the present method formeasuring climatic strains is, however, that the climatically inducedstrains on the power line is measured directly, and not through variableproviding a more or less sufficient basis for calculating them.

The invention consists of a new method making it possible to registerclimatic strains in high voltage air stretches. The method is based onfiber optic sensors, preferably Bragg gratings. This type of measuringsystems are described in a number of different patent publications, forexample U.S. Pat. No. 5,845,033, which describes a fiber optic measuringsystem for monitoring pipes in oil and gas systems. The opticalmeasuring system comprises an optical fiber being wound along a helicalline on the pipe. In its longitudinal direction the optical fiber isprovided with a number of sensors, being Bragg gratings, adapted toreflect light with different wavelengths. A light source emits lightwith a large range of wavelengths into the fiber. As the different Bragggratings reflect light with different wavelengths strain induced changesin the different gratings will indicate the amplitude and the positionof the provided strain as changes in the spectrum of the reflectedlight. This system demands robust solutions because of the verydemanding environment in which it is meant to be used, as the sensor ispositioned on the outside of the pipe. The system also requiresdedicated equipment adapted to the measurements.

In international patent application PCT/US94/00967 a more generaldescription of a known sensor system of the type used in the abovementioned US patent is provided, without giving any specific field ofuse.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a monitoring systemfor power lines which in an exact and cost effective way makes itpossible to monitor under varying weather conditions without positioningsensor equipment locally. This is obtained with a system and a use ofthe equipment as described in the independent claims.

Recently fiber optic cables for telecommunication purposes has beenmounted on high voltage cables. An additional advantage related to thisinvention is that existing equipment may be used to mount and to acertain degree control the measuring system, which reduces the costs andprovides a cost effective system.

The invention will be described below with reference to the accompanyingdrawings, illustrating examples of embodiments.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a span in a high voltage cable with a number ofsensors mounted in chosen positions.

FIG. 2 illustrates schematically an embodiment of a per se known opticalsensor set up.

FIG. 3 shows an embodiment of the invention in which the optical fiberis positioned inside a tube, which preferably is a hollow strand.

FIG. 4 shows an embodiment of a Bragg grating mounted on a surface.

FIG. 5 shows a fiber mounted in a plastic cover line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a high voltage cable of a known type stretched between twopylons 2. An optical fiber 3 is mounted on the power cable 1 and isprovided with a number of sensors comprising Bragg gratings. The sensors4 are positioned on places along the cable which according to experienceare subject to large strains, so that an increase in strain isdiscovered at as early stage as possible. In addition the positioned ofthe sensors are chosen so as to measure only on cable stretches beingespecially exposed, such as long stretches over fjord crossings wherevibrations is a known problem and in exposed mounted regions where icingand galloping are known problems.

The optical fiber will ordinarily be covered by a protecting casing,especially in regions being exposed to weather and wind, and may befastened to the high voltage cables in the same way as optical fibersbeing used in telecommunications, by using equipment developed for thispurpose. It is especially preferred according to the invention that thecable is positioned in a metal free fiber cable, as it is preferred thatthe measuring system is electrically insulated from the high voltageenvironment. According to a preferred embodiment of the invention theoptical fiber 2 is mounted in the high voltage cable Itself during theproduction of this. The latter solution will provide optimal protectionand thus reduce the error sources due to fiber damage.

In FIG. 1 the optical fiber 3 is lead away from the high voltage cable 1to a central measuring station 5, which preferably is positionedtogether with other service equipment and similar for the system, forexample in relation to a transformer station. Because of the low loss inthe optical fiber the central measuring station 5 may be positioned atlocation being relatively far from the cable or cables being monitored.In this drawing the measurements are performed over a single stretch,but measurements over several stretches are of course possible.

In FIG. 2 a known measuring assembly is illustrated for use according tothe invention. In the illustrated measuring assembly a light source 6,for example a laser or a super luminescent diode, emits light within awell defined range of wavelengths into the optic fiber 3 through anoptic coupler 8.

As described in the abovementioned patent publications light will bereflected from the Bragg gratings 4, and the reflected light is leadthrough the coupler 8 toward a detector system 7 measuring the spectrumof the reflected signal. The wavelength of these reflections is uniquelygiven by the period of the grating and thus the strain from thesurrounding structure on each Bragg grating. The effect of the strain onthe Bragg grating is determined beforehand by calibration. This way eachBragg grating will function as a strain sensor. If the reflectionwithout external stimulation of the sensors or Bragg gratings 4 is knownchanges may be used to detect changes in the sensors 4.

Bragg gratings may be provided with different reflectioncharacteristics, for example given by different grating constants, sothat each change may indicate in which sensor and thus which positionalong high voltage cable the change has been.

Alternatively the emitted signal may be pulsed, so that the time ofarrival for the received pulse may indicate the position, but this willrequire some filtering of unwanted signals as there will occur somereflections between the Bragg gratings.

The fiber end 9 may be provided with means to avoid reflections back tothe detector system 7, but since the distance to the end is well definedthis reflection, if the emitted signal is pulsed, is easily removed in aper se known way in the detector system.

As an alternatively to the solution shown in FIG. 2 a number of opticalfibers may be used in which each comprises one or more sensors 4. Whichsolution that is chosen will among other things depend on the costrelated to the handling of a larger number of fibers measured againstthe possibility for separating a large number of sensors In one singlefiber. The illustrated solution will usually be the preferred.

In FIG. 3 an optical fiber 3 is shown comprising five sensors 4,4A,where the optical fiber is mounted in a tube or a strand 10. Four of thesensors 4 are fastened to the tube so that they will be affected by, andthus have the ability to detect, strain in the tube. The sensors 4 maybe fastened to the tube in several ways, preferably with epoxy adhesive.

The fifth sensors 4A is not fastened to the tube and will therefore notbe affected by the strain on the tube. The fifth sensor will, however,be affected in the same way as the others by temperature changes in theoptical fiber and may thus be used as a reference to correct changes dueto the relatively large temperature fluctuations which a high voltagecable may be subject to. A number of such reference sensors 4A may bepositioned in different places along the high voltage cable to provide apossibility for compensating for temperature variations.

By mounting the fiber 3 outside a high voltage cable 1 the referencesensor 4A may be positioned in a suitable casing so as not to beaffected by stretch, and at the same time being protected from otherinfluences such as ice and water.

FIG. 4 illustrates a sensor 4 mounted on the surface of a high voltagecable 1. In this example the optic fibers 3 cladding 13 is removed in alimited area, and the Bragg grating 45 is fastened to the fiber 3 sothat its characteristics is changed in a certain area. A protectinglayer 14 is placed over the uncovered fiber. Preferably both thecladding and the sensor area is covered by an additional protectinglayer, for example plastic, to protect against influences from outside.

There exists a number of methods to make a Bragg grating in an opticalfiber, such as diffusion, use of laser and similar. These solutions arewell known within the technical art and will not be described in anydetail here. The chosen solution does not have any important effect onthe principle of the invention.

For better protection both when being mounted and later the fiber areacontaining the Bragg gratings for strain measurements is glued into athin protective sleeve of metal being shaped according to the curvatureof the line strands. This protective sleeve has a diameter beingapproximately the same as the fiber cable transporting the signals, andboth ends are fastened to the fiber cable. With more Bragg gratings in aseries the protective sleeve may advantageously be made as a splitcylinder with a trace in the center for the fiber with the Bragg gratingand where the fiber, the two parts of the sleeve and the ends of thefiber cable are glued beforehand. This sleeve is glued or molded to thehigh voltage line when being mounted.

FIG. 5 shows a cross section of a high voltage cable 1 comprising wires11 or strands being preferably wound around each other. In addition thehigh voltage cable comprises a tube 10, for example as illustrates inFIG. 3 comprising one or more optical fibers for measuring, also beingwound around the wires 11 so that it adds to the construction of thehigh voltage cable in a natural way. The tube 10 may possibly alsocomprise optical or other conductors suitable for other purposes, forexample telecommunication. The high voltage cable is also covered by aprotective plastic layer 12.

As indicated above the optical fiber or fibers may be fastened to thecable in different ways. If measurements are to be performed on existinghigh voltage cables the fibers may be wound on the outside of the cable,possibly at the same time as or in relation to, installation oftelecommunication lines. The optical fibers may then be glued directlyto the high voltage cable, as shown in FIG. 4, or be positioned in aprotective tube, as shown in FIG. 3. To measure the stretch it is onlynecessary to glue the sensor points, which is the Bragg gratings, to thehigh voltage cable. During production of new high voltage cables thesolution illustrated in FIG. 5 may be chosen, where the tube 10 is woundbetween the strands and the plastic protection, or, if the high voltagecable is not coated with plastic, so that the tube constitutes one ofthe strands in the high voltage cable.

The optical fibers or conductors 3 may be of any known type, usuallystandardized fibers of quarts glass, but other solutions using specialfibers may of course be used in some cases. The other components in themeasuring system are adapted to the fiber regarding compatibility withthe chosen fiber material, e.g. in relation to wavelength of the emittedlight, and characteristics under the different conditions, such astemperature and humidity.

What is claimed is:
 1. System for monitoring high voltage cables in air stretches, comprising at least one optical fiber fastened to the cable, said optical fiber comprising at least one Bragg grating with known reflection characteristics, a light source for transmitting light within a known range of wavelengths into said optical fibre, measuring devices for detection of light reflected from said Bragg grating in the fiber and for recognizing light reflected from each Bragg grating based on their known reflection characteristics; wherein at least one of the fibers is positioned essentially loosely in a protective tube; wherein at least one of said optical fibers which comprises at least one Bragg grating is fastened to the tube essentially at the position of at least one of said Bragg gratings intended as a stress or strain sensor thus allowing the said Bragg grating to be affected by mechanical stress or strain of the high voltage cable, thus forming a mechanical stress or strain sensing Bragg grating sensor; and where said light source and said measuring device are arranged to emit and receive light, respectively, from a substantially same end of said at least one optical fiber.
 2. System according to claim 1, wherein the system comprises one optical fiber being in its longitudinal direction provided with a number of Bragg gratings.
 3. System according to claim 1, wherein at least one of said fibers is mounted in a protective tube.
 4. System according to claim 3, wherein the tube is a fiber optic cable comprising the optical fibre, in which the fiber optic cable is mounted in the high voltage cable during production so as to constitute a hollow strand in the high voltage cable.
 5. System according to claim 3, wherein the tube is a fiber optic cable comprising the optical fiber and being wound around an already suspended high voltage cable.
 6. An optical measuring system comprising at least one optical fiber being mounted in a protective tube, the optical fiber containing at least one Bragg grating with known reflection characteristics at a chosen position along the fiber, the fiber being fastened to the protective tube in this position, the system also comprising a light source for emitting light within a known range of wavelengths into the optical fiber and a measuring devices for detection of light reflected from the Bragg gratings based on their known reflection characteristics, where the optical fibre is fastened to the protecting tube; and where said light source and said measuring device are arranged to emit and receive light, respectively, from a substantially same end of said at least one optical fiber.
 7. The optical measuring system according to claim 6, wherein each Bragg grating is fastened to the high voltage cable at a chosen position for measuring strain in the high voltage cable at this position.
 8. The optical measuring system according to claim 6, wherein the optical fiber is mounted internally in the high voltage cable.
 9. The optical measuring system according to claim 6, wherein the optical fiber comprises an outer casing along essentially its whole length and is wound on the outside of the high tension cable.
 10. The optical measuring system according to claim 6, wherein all the Bragg gratings are mounted in one optical fiber.
 11. A method of monitoring suspended air cables comprising the steps of: providing a monitoring system having at least one optical fibre with at least one Bragg grating of known reflection characteristics; mounting the monitoring system on an already suspended aerial cable; fastening the optical fibre to the suspended cable essentially at the position of said Bragg grating; emitting light from a light source of a known wavelength range into said optical fibre at an end of said optical fiber; reflecting said light with said Bragg grating; detecting light reflected from said Bragg grating; measuring a wavelength of the reflected light at said end of said optical fiber; and measuring the condition of the Bragg gratings.
 12. The method of monitoring suspended air cables of claim 11 wherein the step of fastening the optical fibre to the cable essentially at the position of said Bragg grating comprises: fastening the optical fibre to a protective tube; and fastening the protective tube to the suspended cable. 